A seat used for an airplane, train, ship, automobile or the like is always requested to be improved not only in its fundamental function such as impact absorbency, vibration absorbency, and the like, but also in various functions such as physique difference absorbency, posture difference absorbency, body movement smoothness to assist the above-described functions and to obtain more pleasant riding comfort. Further, in recent years, in order to reduce the weight of transportation machine such as an automobile and the like from the viewpoint of environmental measures by improving fuel efficiency, a seat used for these transportation machine is also required to reduce its weight in addition to improve the above-described various functions, and technique for using a thinner cushioning member lighter in weight and so on has been proposed.
From the above-described viewpoint, for instance, the present applicant proposes a three-dimensional solid knitted fabric having light weight but characteristics (spring characteristic and damping characteristic) sufficient as a cushioning member for a seat structure by straining a solid knitted fabric having several millimeters to several tens of millimeters in thickness over frames to use as a tension structure (refer to Patent Document 1: Japanese Patent Application Laid-open No. 2002-339206). The three-dimensional solid knitted fabric disclosed in Patent Document 1 is a solid knitted fabric set with two or more portions different in compression rate, and by using the solid knitted fabric strained over a cushion frame or a back frame in order to use as a tension structure, a spring characteristic similar to the spring characteristic of the muscle of human haunches or the like can be exhibited. As a result, it becomes possible to prevent deformation of the muscle of haunches or the like at the time when a person takes a seat, to lower a counter force from a cushioning member to a person, and to reduce the bloodstream trouble or a muscle load.
In other words, the cushioning member provided with such characteristics can be said to function as a portion of a pseudo-muscle layer, but in order to provide the cushioning member with such functions, it is necessary to strain the cushioning member over the cushion frame or the back frame with a low elongation percentage, preferably in the range of 0 to 5%. Accordingly, when a cushioning member is used individually to a cushion frame, it lacks a feeling of stroke and is apt to give a feeling of bottom touch at the time of being seated. Further, it is necessary to take measures to absorb vibration coming from a vehicle body floor. As a structure for this purpose, the present applicant proposes a seat structure in which a flat-type supporting member made of Plumaflex, cloth spring, or the like disposed under a cushioning member to be mounted on the seat cushion is elastically arranged to a side frame of the cushion frame via a coil spring, as disclosed in Patent Document 2 (Japanese Patent Application Laid-open 2002-177099). This structure exhibits a high vibration absorbing characteristic by movement of the flat-type supporting member and the coil spring. However, as described above, a seat for transportation machine is always requested to be improved in vibration absorbing characteristic and the like.